With respect to this type of caster, there is a conventional caster as shown in FIGS. 8 and 9, which includes double-wheels. A main body 11 has an axial bore 12 at a lower part thereof into which a wheel axis 14 for wheels 13 is inserted. The wheel axis 14 is supported by ball bearings 15 provided on the wheels 13 so as to rotatably mount the double wheels 13 on both sides of the main body 11.
In the above double-wheel caster, the wheel axis 14 is in general a bolt. Both end parts of the bolt, namely a head part and an axis end, are exposed in the central parts of the wheels. Furthermore parts of the ball bearings 15 which pivotally support the wheel axis 14 are exposed at the central parts of the wheels 13. They deteriorate the appearance of the caster.
For that, in the double-wheel caster, as shown in the drawings, a stopper 18 of a cover 17 is engaged with an engaging hole 16 passing through from an exterior face 13a to an interior face 13b of each of the wheels 13, and thus the cover 17 is attached to the exterior face 13a of the wheel 13 and covers the both end parts and the ball bearings 15 exposed in the central part of the wheel 13. An example of such a caster is described in the Japanese Utility Model Application, Laid Open No. H5-86602.
When the foregoing caster is mounted on a shopping cart, furniture and other carriage, the load thereof is applied on the wheel axis 14. A large load would necessitate a strong and thick wheel axis and this consequently necessitates bigger ball bearings 15, which inevitably enlarges the exposed amount and further worsens the appearance of the caster. Even though the cover 17 is used to hide the both end parts of the wheel axis 14 and ball bearings 15, which just does not worsen the appearance only, but not improve it.
Recently in order to improve appearance of a double-wheel caster, there is an idea to eliminate a wheel axis and provide a large opening at the central part of a wheel. When light passes through the opening, the wheel becomes less showy, by which a member on which a caster is mounted, such as a shopping cart, furniture and carriage, appears “being floating.” This double-wheel caster, for example shown in FIGS. 10 and 11, includes a ring body 21 and ring shaped wheels 22 which are rotatably mounted on the ring body 21.
The ring body 21 has, on its outer circumference, a boss 23 outwardly provided as one body and a support pin 24 provided on the boss. The support pin is used to be mounted on a shopping cart, furniture and other carriage. The body 21 has a central opening part 25 with a large diameter.
The ring shaped wheel 22 has an inner circumferential flange 26. Plural roller bearings 28 are disposed between the inner circumferential flange 26 and a neck part 27a of an annular holder 27. A circumferential end of the neck part 27a of the holder 27 is fixed within the opened central part 25 so that the ring shaped wheel 22 is rotatably attached to the ring body 21. This example is described in Japanese Patent Application, Laid Open No. 2008-526603.
However, the conventional double-wheel casters stated above employ ball bearings 15 or roller bearings 28, and in either case rolling bearings are employed. Rolling bearings provide lower mechanical stability. For the long term use, general maintenance such as oiling is necessary and handling is cumbersome.
Furthermore, in the above conventional double-wheel casters, the used rolling bearings lack of impact-resistance and heat resistance. Therefore if the casters are used under an environment where impact is easily applied or under high temperatures, the casters are unlikely to withstand the usage thereunder.
In addition, the employed rolling bearings in the foregoing double-wheel casters have a large volume, and the casters with these rolling bearings become large in volume. Therefore it is difficult to make them smaller and lighter.
It is hence an object of the present invention to solve the above conventional problems and to provide a caster which has a high mechanical stability, durability, impact resistance and heat resistance, requires no maintenance, withstands the use under environment where impact is readily applied and/or under high temperatures, and furthermore can be readily reduced in size and weight.
A caster according to the present invention includes a ring-shaped main body, two ring-shaped wheels provided on both side end parts of the main body, and two ring-shaped holders by which the two ring-shaped wheels are rotatably fitted on the main body. Each of the wheels has a first sliding face on an inner side of an inner circumferential part and a second sliding face on an outer side thereof. A first dry bearing is provided between an outer circumferential face of a ring-shaped flange provided on each of the both side end parts of the main body and the first sliding face. A second dry bearing is provided between an outer circumferential face of a ring-shaped flange provided on an outer end part of each of the holders and the second sliding face).
In the caster according to the present invention, the holders have respective connecting parts on their inner circumferential end parts, and the connecting parts are linked with each other in an inner circumferential part of the main body.
In the caster according to the present invention, the holders may have respective connecting parts on their inner circumferential end parts, and the connecting parts are respectively fixed on an inner circumferential part of the main body.
In the caster according to the present invention, at least one of the dry bearings is formed in a ring-shape.
In the caster according to the present invention, at least one of the dry bearings may be formed in a ring-shape with a cut.
In the caster according to the present invention, at least one of the dry bearings may have plural segments which form a ring-shape.
In the caster according to the present invention, at least one of the dry bearings may be formed in a semi-ring shape.
Constituted as above, the caster according to the present invention has a high mechanical stability, durability, impact resistance and heat resistance, requires no maintenance, withstands the use under environment where impact is readily applied and/or under high temperatures, and furthermore can be readily reduced in size and weight.